In general, various hydraulic actuators are provided in construction machines such as excavators. As a control circuit for controlling supply and discharge of oil to and from such a hydraulic actuator, a control circuit has been known which is configured to simultaneously perform, using one spool valve, directional control that switches between the direction of supply of hydraulic oil to the hydraulic actuator and the direction of discharge of hydraulic oil from the hydraulic actuator, meter-in control that controls a supply flow rate from a hydraulic pump to the hydraulic actuator, and meter-out control that controls a discharge flow rate from the hydraulic actuator to an oil tank. However, when the meter-in control and the meter-out control are thus performed using one spool valve, a relation between a meter-in opening area and a meter-out opening area with respect to a moving position of the spool valve is uniquely determined. This causes a problem in that the relation between the meter-in and the meter-out cannot be changed in accordance with the contents of work and an operator.
Thus, a technique has been known in which the control of supply and discharge of oil to and from the hydraulic actuator is performed by a bridge circuit formed using four metering valves including a head-side meter-in valve and a rod-side meter-in valve (a head-end supply valve and a rod-end supply valve) that control supply flow rates from the hydraulic pump to a head-side oil chamber of a hydraulic cylinder and to a rod-side oil chamber of the hydraulic cylinder, respectively, and a head-side meter-out valve and a rod-side meter-out valve (a head-end drain valve and a rod-end drain valve) that control discharge flow rates from the head-side oil chamber and from the rod-side oil chamber to the oil tank, respectively (see, for example, PTL 1). In this technique, the four metering valves are individually actuated based on commands from a controller. Thus, the relation between the meter-in and the meter-out can be easily changed in accordance with the contents of work and the operator.
Moreover, the bridge circuit using the four metering valves enables recycling such that discharged oil from one of the oil chambers of the hydraulic actuator is supplied to the other oil chamber. When the recycling is performed, both the head-side and rod-side meter-in valves are opened to merge the discharged oil from one of the oil chambers with pump discharge oil via one of the meter-in valves, and the merged hydraulic oil is then supplied to the other oil chamber via the other mere-in valve.
On the other hand, as another technique that individually performs the meter-in control and the meter-out control on the hydraulic actuator, a technique is also known in which a meter-in selector valve that switches a port of the hydraulic actuator connected to the hydraulic pump while controlling a supply flow rate to the port and a meter-out selector valve that switches a port of the hydraulic actuator connected to the oil tank while controlling a discharge flow rate from the port are provided, and the meter-in selector valve and the meter-out selector valve are independently controlled (see, for example, PTL 2).